Treatment fluids containing a boron trifluoride complex and methods for use thereof

ABSTRACT

Treatment fluids for use in subterranean formations, particularly sandstone and other siliceous formations, may contain a source of fluoride ions to aid in mineral dissolution. In some cases, it may be desirable to generate the fluoride ions from a fluoride ion precursor, particularly a hydrofluoric acid precursor, such as a boron trifluoride complex. Methods for treating a subterranean formation can comprise providing a treatment fluid that comprises an aqueous base fluid, a boron trifluoride complex, and a chelating agent composition, and introducing the treatment fluid into a subterranean formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/094,248, filed Apr. 26, 2011, which is acontinuation-in-part of U.S. patent application Ser. No. 13/051,827,filed Mar. 18, 2011, which is a continuation-in-part of U.S. patentapplication Ser. No. 11/499,447, filed Aug. 4, 2006, each of which isincorporated herein by reference in its entirety whether or notexpressly set forth herein.

BACKGROUND

The present invention generally relates to treatment fluids used forstimulation and remediation operations in subterranean formations, and,more particularly, to treatment fluids that contain a boron trifluoridecomplex and methods for using such treatment fluids.

Treatment fluids can be used in a variety of subterranean operations.Such subterranean operations can include, without limitation, drillingoperations, stimulation operations, production operations, remediationoperations, sand control treatments and the like. As used herein, theterms “treat,” “treatment,” and “treating” refer to any subterraneanoperation that uses a fluid in conjunction with achieving a desiredfunction and/or for a desired purpose. Use of these terms does not implyany particular action by the treatment fluid. Illustrative treatmentoperations can include, for example, fracturing operations, gravelpacking operations, acidizing treatments, scale dissolution and removal,consolidation treatments, and the like. In alternative embodiments,treatment operations can refer to an operation conducted in a pipe,tubing, or like vessel in conjunction with achieving a desired functionand/or for a desired purpose (e.g., scale removal).

In acidizing treatments, for example, subterranean formations comprisingacid-soluble components, such as carbonate and sandstone formations, canbe contacted with a treatment fluid comprising an acid to dissolve theformation matrix. After the acidizing treatment is completed, thetreatment fluid and salts dissolved therein can be recovered byproducing them to the surface (e.g., “flowing back” the well), leaving adesirable amount of voids or conductive pathways (e.g., wormholes incarbonates) within the formation. Acidizing operations can enhance theformation's permeability and can increase the rate at which hydrocarbonsare subsequently produced from the formation.

Acidizing a siliceous formation (e.g., a sandstone formation, aclay-containing formation or a like aluminosilicate-containingformation) can introduce certain challenges that are not present whenacidizing a carbonate formation. As used herein, the term “siliceous”refers to the characteristic of having silica and/or a silicate,including aluminosilicates. Most sandstone formations are composed ofabout 40% to about 98% sand quartz particles, i.e, silica (SiO₂), bondedtogether by varying amounts of a cementing material including carbonate(calcite or CaCO₃), aluminosilicates, and silicates. Carbonateformations can usually be effectively treated with a variety of acidsystems, including mineral acids (e.g., hydrochloric acid) and organicacids (e.g., acetic and formic acids), often with similar success, wherethe acidity of the treatment fluid alone can be sufficient to solubilizeformation cations. The treatment of siliceous formations with theseacids, however, can have little or no effect because most organic andmineral acids do not react appreciably with the siliceous mineralscharacterizing these formations.

By far the most common method of treating sandstone and other siliceousformations involves introducing corrosive, very low pH fluids comprisinghydrofluoric acid into the well bore and allowing the acid to react withthe formation matrix. In contrast to other mineral acids, hydrofluoricacid can be very reactive with aluminosilicates and silicates (e.g.,sandstone, clays and feldspars). In some cases, hydrochloric acid can beused in addition to hydrofluoric acid in the treatment fluid to maintaina low pH as hydrofluoric acid becomes spent during a treatmentoperation, thereby retaining certain dissolved species in a highlyacidic solution. Hydrofluoric acid acidizing can often be used to removedamage that is present within the subterranean formation.

Although treatment fluids containing hydrofluoric acid and, optionally,another acid can be desirably used to affect dissolution of siliceousminerals, the use of low pH fluids can have detrimental consequences incertain instances. Specifically, at low pH values, dissolved fluorideions can precipitate and damage the subterranean formation, particularlyin the presence of certain cations such as, for example, Al³⁺, Group 1metal ions (e.g., Na⁺ and K⁺) and/or Group 2 metal ions (e.g., Mg²⁺,Ca²⁺, and Ba²⁺). In some cases, this precipitation can damage theformation more than if the original treatment operation had not beenperformed at all. For instance, hydrofluoric acid tends to react veryquickly with authigenic clays (e.g., smectite, kaolinite, illite andchlorite), especially at temperatures above 200° F. and below pH 1, as afunction of mineral surface area. Because of this rapid reaction, thehydrofluoric acid can penetrate only a short distance into the formationbefore becoming spent. Simultaneously, precipitation of various aluminumand silicon compounds can occur as increasing amounts of siliceousminerals are dissolved at low pH. At certain temperatures andsubterranean conditions, dissolution of a sandstone matrix or likesiliceous material can sometimes occur so rapidly that uncontrolledprecipitation can become an inevitable problem. The precipitatedproducts can plug pore spaces and reduce the porosity and permeabilityof the formation, thus impairing flow potential. In addition, low pHtreatment fluids containing one or more acids can present corrosion andsafety issues.

The precipitation of calcium fluoride, fluorosilicates, and otherinsoluble fluoro compounds during hydrofluoric acid treatments can be ofparticular concern, since production can be delayed while damage controloperations are conducted. Fluorosilicates can be especially problematicbecause they are the primary product of the dissolution of a clay andhydrofluoric acid. In addition, fluorosilicates can be difficult toremediate through redissolution. Calcium fluoride can be a later concernin the process, because the fluoride anion needs to be present in itsfree ion form, and that does not happen until a higher pH is reachedafter some of the acid becomes spent. Unlike fluorosilicates, calciumfluoride can be remediated, in some instances Remediation techniques caninclude a commercially available treatment system from HalliburtonEnergy Services, Inc. known as “F-SOL” acid system, which can be used todissolve calcium fluoride. Fluoroaluminate formation can also be of asignificant concern due to the reaction of fluorosilicates with clayminerals. Fluoroaluminates are thought to be soluble as long as the pHis below about 2 and the ratio of F/Al is maintained below about 2.5. Ifprecipitated, their dissolution typically requires strong HCl (>5%).

Avoiding the formation of calcium fluoride, fluorosilicates, or otherinsoluble fluoro compounds can be a primary design objective in atreatment operation conducted in a subterranean formation or elsewhere.Various means have been used with mixed success to accomplish theforegoing. Blends of organic acids and hydrofluoric acid can be used toslow the dissolution kinetics of sandstone formation solids. However,since organic acids typically have higher pKa values than do mineralacids, precipitation can become problematic as the treatment fluid's pHrises. Pre-flush sequences with mineral acids can be used to removecalcium salts from sandstone formations, before the main acidizingsequence is conducted to remove formation aluminosilicates. Generally,these flushes do not spend completely and typically return, uponflowback, with a persisting low pH. In addition to presenting safetyissues, the return of an acidic fluid can result in corrosion ofdownhole tubular goods (including coiled tubing) and surface equipment.Other multi-stage sandstone acidizing treatment operations can also beused, particularly to remove calcium ions.

Chelating agents can also be included in treatment fluids to sequesterat least a portion of the formation cations that cause unwantedprecipitation. Likewise, chelating agents can be used in treatingpipelines, tubing, and like vessels by removing metal ion scale from thepipeline or tubing surface. However, there are certain operationalconcerns that can be encountered with the use of many common chelatingagents. First, many common chelating agents are not biodegradable orraise toxicity issues that can make their use in a subterraneanformation problematic. Further, the available salt form of somechelating agents can actually exacerbate precipitation problems in atreatment operation rather than lessening precipitation.

Although chelating agents can extend the conditions under whichtreatment fluids containing hydrofluoric acid can be effectively used,even better precipitation control over a wider range of pH values, whilestill achieving a satisfactory dissolution rate of siliceous materials,would be highly desirable from an operational standpoint. Furthermore,in terms of safety and ease of handling, it would also be desirable tobe able to affect dissolution of siliceous materials without resortingto the use of highly acidic treatment fluids.

SUMMARY OF THE INVENTION

The present invention generally relates to treatment fluids used forstimulation and remediation operations in subterranean formations, and,more particularly, to treatment fluids that contain a boron trifluoridecomplex and methods for using such treatment fluids.

In some embodiments, the present invention provides a method comprising:providing a treatment fluid that comprises: an aqueous base fluid; aboron trifluoride complex; and a chelating agent composition; andintroducing the treatment fluid into a subterranean formation.

In some embodiments, the present invention provides a method comprising:providing a treatment fluid that comprises: an aqueous base fluid; aboron trifluoride complex; and a chelating agent composition;introducing the treatment fluid into a subterranean formation having atemperature of at least about 200° F.; allowing sufficient time to passfor at least a portion of the boron trifluoride complex to formhydrofluoric acid in the subterranean formation; and dissolving at leasta portion of any insoluble silicon-containing compounds present in thesubterranean formation using the treatment fluid.

In some embodiments, the present invention provides a method comprising:providing a treatment fluid that comprises: an aqueous base fluid; aboron trifluoride complex; and a chelating agent composition; anddissolving at least a portion of any insoluble silicon-containingcompounds present in a subterranean formation having a temperature of atleast about 200° F. by using the treatment fluid.

In some embodiments, the present invention provides a treatment fluidcomprising: an aqueous base fluid; a boron trifluoride complex; and achelating agent composition; wherein the treatment fluid has a pH ofabout 2 or greater.

The features and advantages of the present invention will be readilyapparent to one of ordinary skill in the art upon a reading of thedescription of the preferred embodiments that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent invention, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modification,alteration, and equivalents in form and function, as will occur to onehaving ordinary skill in the art and having the benefit of thisdisclosure.

FIG. 1 shows an illustrative plot of the concentration of various ionsin fractional volumes collected from a flow test of a synthetic corepack of quartz and chlorite at 245° F. using a treatment fluidcontaining 0.17% boron trifluoride dihydrate complex and 5% TRILON® M atpH 3.68.

FIG. 2 shows an illustrative plot of the concentration of silicon,aluminum, and boron in fractional volumes collected from a flow test ofa synthetic core pack containing 10% Kaolinite and 90% quartz (55% SSA-1and 45% 20/40 Brady Sand) at 240° F. using a treatment fluid containing0.4% boron trifluoride dihydrate complex and 5% TRILON® M at pH 3.8.

FIG. 3 shows an illustrative plot of the concentration of silicon,aluminum, and boron in fractional volumes collected from a flow test ofa synthetic core pack containing 15% Kaolinite and 85% quartz (55% SSA-1and 45% 20/40 Brady Sand) at 245° F. using a treatment fluid containing0.4% boron trifluoride dihydrate complex and 15% TRILON® M at pH 3.45.

FIG. 4 shows an illustrative ¹¹B NMR spectrum of boron trifluoridedihydrate complex at pH 2 in a solution containing TRILON® M.

FIG. 5 shows an illustrative ¹¹B NMR spectrum of boron trifluoridedihydrate complex at pH 3.8 in a solution containing TRILON® M.

FIG. 6 shows a comparative ¹¹B NMR spectrum of a 12.5 wt. % fluoroboricacid solution at pH 5.

DETAILED DESCRIPTION

The present invention generally relates to treatment fluids used forstimulation and remediation operations in subterranean formations, and,more particularly, to treatment fluids that contain a boron trifluoridecomplex and methods for using such treatment fluids.

Treatment fluids described herein employ a boron trifluoride complex asa precursor for the formation of hydrofluoric acid. A number ofadvantages may be realized by using a boron trifluoride complex as ahydrofluoric acid source in a treatment fluid instead of using otherhydrofluoric acid-generating compounds (e.g., HBF₄, bifluoride salts,and the like) or hydrofluoric acid directly. First, the handling ofhighly corrosive and toxic hydrofluoric acid may be avoided, whichoffers numerous safety advantages when formulating and/or using thetreatment fluids. Second, the use of boron trifluoride complexes allowshigher pH treatment fluids (e.g., a pH of about 2 or greater) to beused, which may lessen the risk of unwanted precipitation. Finally, anumber of different boron trifluoride complexes are well known, each ofwhich may release hydrofluoric acid at a different rate depending on itsstability in an aqueous fluid, which is dependent upon the ligandcomplexed to boron. In at least some embodiments, the differentialstabilities of the boron trifluoride complexes may advantageously beutilized to affect release of hydrofluoric acid at a desired time or ata desired location within a subterranean formation.

Although boron trifluoride may be substituted for a boron trifluoridecomplex in any of the present embodiments, such substitution may be lesspreferable than using a boron trifluoride complex. The chief reason forthe preferred use of a boron trifluoride complex is that borontrifluoride is a highly reactive and toxic gas. Boron trifluoridecomplexes, in contrast, are either liquids or solids that are much moreeasily handled than is ligand-less boron trifluoride. The handling ofboron trifluoride complexes is well known in synthetic chemistry, andthe comparatively stable nature of these compounds facilitates their useas a readily manipulated source of hydrofluoric acid. As previouslynoted, the boron trifluoride complex's ligand may also control the rateof hydrofluoric acid formation, which is not possible through the directuse of hydrogen fluoride gas.

The treatment fluids described herein can generally comprise an aqueousbase fluid, a boron trifluoride complex, and a chelating agentcomposition. In some embodiments, the treatment fluid may have a pH ofabout 2 or greater. Keeping the treatment fluid's pH at about 2 orgreater may avoid precipitation that sometimes takes place at lower pHvalues. Use of such a pH range may also lessen corrosion issues that mayotherwise be encountered.

In some embodiments, the treatment fluids can further comprise an acid.In some embodiments, the acid can comprise a mineral acid such as, forexample, hydrochloric acid. In some embodiments, the acid can comprisehydrofluoric acid. In some embodiments, the hydrofluoric acid may beproduced in situ from a hydrofluoric acid-generating compound.

The generation of HBF₃OH from a solution containing H₃BO₃ necessitatesthe presence of HF in order to produce a BF₄ ⁻ anion. A BF₃ solutiondoes not generate HF unless it undergoes a reaction that drives theformation of BF₄ ⁻, an event that does not take place spontaneously.Furthermore, BF₃ may adsorb to a metal oxide surface when in solutionand fail to undergo hydrolysis. A particular aspect of HBF₄ solutions istheir rapid hydrolysis to generate HF and possibly other species. Thehydrolysis rate of boron trifluoride complexes is not as rapid.Conventional equilibria of HF/H₃BO₃/HCl solutions producing HBF₄indicate that the formation of HBF₃OH species is very fast.

In the embodiments described herein, a boron trifluoride complex may beused as a hydrofluoric acid-generating compound in a treatment fluid.When used in a treatment fluid, the ligand may be released into thetreatment fluid and the boron trifluoride may hydrolyze to producehydrofluoric acid, boric acid, fluoroboric acid, and possibly otherspecies. A distinct advantage of boron trifluoride complexes in thisregard is that their rate of hydrolysis is slow compared to a solutionof HBF₄. Suitable boron trifluoride complexes for use in the presentembodiments may include, for example, boron trifluoride acetonitrilecomplex, boron trifluoride acetic acid complex, boron trifluoridedimethyl ether complex, boron trifluoride diethyl ether complex, borontrifluoride dipropyl ether complex, boron trifluoride dibutyl ethercomplex, boron trifluoride t-butyl methyl ether complex, borontrifluoride phosphoric acid complex, boron trifluoride dihydrate, borontrifluoride methanol complex, boron trifluoride ethanol complex, borontrifluoride propanol complex, boron trifluoride isopropanol complex,boron trifluoride phenol complex, boron trifluoride propionic acidcomplex, boron trifluoride tetrahydrofuran complex, boron trifluoridepiperidine complex, boron trifluoride ethylamine complex, borontrifluoride methylamine complex, boron trifluoride triethanolaminecomplex, any derivative thereof, and any combination thereof.

Some of the foregoing boron trifluoride complexes may be more suitablefor certain applications than for others. As will be recognized by onehaving ordinary skill in the art, the rate at which hydrofluoric acid isgenerated from the boron trifluoride complexes may be determined, atleast in part, by the identity of the ligand complexed to the boron.These ligands may be released into the treatment fluid as borontrifluoride decomposes to produce hydrofluoric acid. The in situaddition of the free forms of these ligands to the treatment fluid maybe advantageous in certain applications. For example, solvents (e.g.,acetonitrile, alcohols and ethers), acids (e.g., phosphoric acid andacetic acid) and amines (e.g., methylamine and ethylamine) may bereleased into the treatment fluid from various boron trifluoridecomplexes, which may further enhance the effectiveness of a treatmentoperation in some instances.

In some embodiments, other hydrofluoric acid-generating compounds may beused in conjunction with a boron trifluoride complex. Reasons one mightchoose to include another hydrofluoric acid-generating compound in thetreatment fluid in addition to the boron trifluoride complex is if therate of hydrofluoric acid generation is not sufficiently high. Forexample, if the temperature of a subterranean formation is less thanabout 200° F., the rate of hydrofluoric acid generation from a borontrifluoride complex may become undesirably slow. Illustrativehydrofluoric acid-generating compounds that may be used in conjunctionwith boron trifluoride complexes in the present embodiments may include,for example, fluoroboric acid, fluorosulfuric acid, hexafluorophosphoricacid, hexafluoroantimonic acid, difluorophosphoric acid,hexafluorosilicic acid, potassium hydrogen difluoride, sodium hydrogendifluoride, polyvinylarnmonium fluoride, polyvinylpyridinium fluoride,pyridinium fluoride, imidazolium fluoride, ammonium fluoride, ammoniumbifluoride, tetrafluoroborate salts, hexafluoroantimonate salts,hexafluorophosphate salts, bifluoride salts, and any combinationthereof.

In various embodiments, the boron trifluoride complex may be present inthe treatment fluid in an amount ranging between about 0.1% to about 10%by weight of the treatment fluid. In other embodiments, an amount of theboron trifluoride complex may range between about 0.5% and about 10% orbetween about 0.5% and about 5% by weight of the treatment fluid.

The treatment fluids described herein may also use a chelating agent inconcert with the boron trifluoride complex. Although their use may beoptional in some embodiments, a chelating agent may help maintaindissolved cations in solution and/or sequester cations that promoteprecipitation of dissolved cations. For example, a chelating agent maybe used to sequester aluminum cations which may otherwise form aprecipitate in the presence of dissolved silicon.

A great number of chelating agents will be well known to one havingordinary skill in the art. In general, any chelating agent may be usedin conjunction with the present embodiments. In some embodiments,suitable chelating agents may include compounds such as, for example,ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA),hydroxyethylethylenediaminetriacetic acid (HEDTA),diethylenetriaminepentaacetic acid,ethylenediaminedi(o-hydroxyphenylacetic) acid, glucoheptonic acid,gluconic acid, any salt thereof, any derivative thereof, and the like.The foregoing list of chelating agents is meant to be for purposes ofillustration and not limitation, and other suitable chelating agents mayreadily be envisioned by one having ordinary skill in the art. In someembodiments, particularly advantageous chelating agents are those thatare biodegradable.

As used herein, the term “biodegradable” refers to a substance that maybe broken down by exposure to environmental conditions including nativeor non-native microbes, sunlight, air, heat, and the like. Use of theterm “biodegradable” does not imply a particular degree ofbiodegradability, mechanism of biodegradability, or a specifiedbiodegradation half-life. Suitable biodegradable chelating agents foruse in the present embodiments may include, without limitation, glutamicacid diacetic acid (GLDA), methylglycine diacetic acid (MGDA), β-alaninediacetic acid (β-ADA), ethylenediaminedisuccinic acid,S,S-ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDS),hydroxyiminodisuccinic acid (HIDS), polyamino disuccinic acids,N-bis[2-(1,2-dicarboxyethoxy)ethyl]glycine (BCA6),N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid (BCA5),N-bis[2-(1,2-dicarboxyethoxy)ethyl]methylglycine (MCBA5),N-tris[(1,2-dicarboxyethoxy)ethyl]amine (TCA6), N-methyliminodiaceticacid (“MIDA”), iminodiacetic acid (IDA), N-(2-acetamido)iminodiaceticacid (ADA), hydroxymethyl-iminodiacetic acid, 2-(2-carboxyethylamino)succinic acid (CEAA), 2-(2-carboxymethylarnino) succinic acid (CMAA),diethylenetriamine-N,N″-disuccinic acid,triethylenetetramine-N,N′″-disuccinic acid,1,6-hexamethylenediamine-N,N′-disuccinic acid,tetraethylenepentamine-N,N′″-disuccinic acid,2-hydroxypropylene-1,3-diamine-N,N′-disuccinic acid,1,2-propylenediarnine-N,N′-disuccinic acid,1,3-propylenediamine-N,N′-disuccinic acid,cis-cyclohexanediamine-N,N′-disuccinic acid,trans-cyclohexanediamine-N,N-disuccinic acid,ethylenebis(oxyethylenenitrilo)-N,N-disuccinic acid, glucoheptanoicacid, cysteic acid-N,N-diacetic acid, cysteic acid-N-monoacetic acid,alanine-N-monoacetic acid, N-(3-hydroxysuccinyl) aspartic acid,N-[2-(3-hydroxysuccinyl)]-L-serine, aspartic acid-N,N-diacetic acid,aspartic acid-N-monoacetic acid, including any salt, any derivative, orany combination of these chelating agents. In some embodiments, abiodegradable chelating agent may be used in combination with achelating agent that is not biodegradable.

In some embodiments, the chelating agents may be used in an ammonium,tetraalkylammonium, or tetraalkylphosphonium salt form, which may beparticularly advantageous for treatment operations conducted using atreatment fluid containing hydrofluoric acid or a hydrofluoricacid-generating compound (e.g. a boron trifluoride complex). Use ofthese salt forms may avoid the additional precipitation problems thatmay sometimes occur when other salt forms (e.g., alkali metal salts) areused in the context of this disclosure. For example, sodium ions from achelating agent may lead to formation of sodium fluorosilicateprecipitates under certain conditions. Such precipitation issues are notbelieved to be problematic in the presence of ammonium,tetraalkylammonium or tetraalkylphosphonium salts.

Furthermore, treatment fluids containing a boron trifluoride complexand, optionally, a chelating agent may be used in prevention embodimentsto inhibit the formation of precipitates, as discussed above, as well asremediation embodiments to remove precipitation damage in a well bore orsubterranean formation. These features may beneficially allow suchtreatment fluids to perform single stage treatment operations including,for example, acidizing treatments (e.g., matrix acidizing), stimulationtreatments, and proppant pack treatments. Likewise, beneficial effectsmay also be observed when treating a pipe, tubing, or like vessel, evenwhen the pH is not particularly low.

It is to be recognized that in certain embodiments, the chelating agentcomposition may be omitted from the treatment fluids. In suchembodiments, dissolution may take place via the hydrofluoric acidgenerated from the boron trifluoride complex. In addition to the borontrifluoride complex and optional chelating agent composition, salts,other pH additives, corrosion inhibitors, surface active agents,anti-sludging agents, mutual solvents, scale inhibitors, viscosifiers,gases, diverting/fluid loss agents, and the like may optionally beincluded in the treatment fluids. The present treatment fluids may beused in subterranean formations to prevent or remediate precipitationdamage in the formation caused by the dissolution of formation cations.Likewise, the present treatment fluids may be used in treating pipes,tubing, and like vessels.

Generally, the base fluid of the treatment fluid may comprise anyaqueous fluid or non-aqueous fluid. In some embodiments, the base fluidmay comprise fresh water, salt water (e.g., water containing one or moresalts dissolved therein), brine (e.g., saturated salt water), sea water,glycol, any combination thereof, or any derivative thereof. In otherembodiments, the base fluid may comprise a liquid chelating agent orscale control agent by itself. Generally, the base fluid may be from anysource, provided that it does not contain components that mightadversely affect the stability and/or performance of the treatmentfluid.

The chelating agent compositions of the present invention generally maycomprise a chelating agent, any salt thereof, or any derivative thereof.Generally, any chelating agent may be used in the present embodiments,although in some embodiments, the chelating agent may be biodegradable.Examples of suitable derivatives of chelating agents may include estersand alkylated derivatives, for instance. Generally, any derivative maybe used, provided that the derivative still maintains an affinity forbinding metal cations. Examples of suitable salts of the chelatingagents may include sodium salts, rubidium salts, lithium salts,potassium salts, cesium salts, and ammonium salts, includingtetraalkylammonium salts and tetraalkylphosphonium salts. Mixed saltforms may also be used, if desired. As previously noted, use of theammonium, tetraalkylammonium, or tetraalkylphosphonium salts may beparticularly advantageous in certain embodiments of the presentdisclosure.

GLDA is a biodegradable chelating agent that is manufactured from areadily biodegradable, renewable, and human-consumable raw material,monosodium glutamate. GLDA may chelate metal ions such as, but notlimited to, calcium, iron, aluminum, and magnesium over a wide pH rangeand is highly soluble in aqueous treatment fluids. At present, GLDA iscommercially available in its sodium salt form. Other salt forms may beavailable non-commercially, or in smaller quantities, or may be madethrough an ion-exchange technique discussed below. In some embodimentsherein, a preferred form of the GLDA is not the monovalent metal saltform (i.e., an alkali metal salt), but rather an ammonium,tetraalkylammonium, or tetraalkylphosphonium salt of GLDA. A suitablecommercial source of GLDA is a 47 wt. % aqueous solution from Akzo-NobelCorp. available under the tradename DISSOLVINE®.

MGDA is also commercially available in its sodium salt form. A suitablecommercial source of MGDA is a 40 wt. % aqueous solution of the sodiumsalt form, sold by BASF under the tradename TRILON® M.

For use in embodiments in which a sodium salt of GLDA, MGDA, or anyother chelating agent is available, it may be desirable, in some cases,to exchange the sodium cations for other cations such as, for example,potassium, ammonium, tetraalkylammonium, or tetraalkylphosphoniumcations. As discussed above, an ammonium, tetraalkylammonium, ortetraalkylphosphonium salt may be particularly advantageous fortreatment operations conducted in siliceous formations including, forexample, clays and sandstones. Exchange of the sodium cations for othercations may avoid precipitation of compounds such as, for example,NaHSiF₅. Cation exchange is contemplated to take place under conditionsthat will be familiar to one having ordinary skill in the art. Methodsfor exchanging sodium cations for potassium, ammonium,tetraalkylammonium or tetraalkylphosphonium cations are contemplated toinclude, without limitation, ion exchange chromatography and selectiveprecipitation techniques. Other means for exchanging the sodium cationsmay also be envisioned by one having ordinary skill in the art. Asdiscussed further herein, it is contemplated that exchange of at least aportion of the sodium cations may produce better solubility properties,in addition to reducing the risk of unwanted precipitation during use ofthe present treatment fluids.

Lesser concentrations of the free acid form of the chelating agent maybe produced under acidic conditions by diluting the acid in anappropriate volume of water. The amount to include will depend on thespecific minerals and quantity present in the subterranean formation,and the purpose of use and desired pH of the chelating agentcomposition. Exemplary ranges are discussed below. In some embodiments,the pH window for clays may be about 1 to about 6 or about 2 to about 6.In other embodiments, the pH window for clays may be about 1.6 to about4.5. In other embodiments, the pH window for clays may be about 1.5 toabout 1.8, and in other embodiments about 1.6 to about 3, In someembodiments, the treatment fluid may have a pH ranging between about 1.5and about 5, and in other embodiments, the treatment fluid may have a pHranging between about 1.5 and about 3. In some embodiments, thetreatment fluid may have a pH of about 2 or greater. In someembodiments, the treatment fluid may have a pH ranging between about 2and about 6. At the low end of these ranges and below, the chelatingagent may become ineffective for coordinating formation cations, asdiscussed below.

In embodiments in which a pipe, tubing, or like vessel is treated withthe treatment fluids, higher pH values may be more advantageous due tothe possibility of corrosion occurring at lower pH values, particularlybelow a pH of about 2. In some embodiments, the pH for treating a pipe,tubing, or like vessel may range between about 2 and about 6 or betweenabout 5 and about 10. In other embodiments, the pH may range betweenabout 5 and about 8 or between about 6 and about 8. In still otherembodiments, the pH may be greater than about 8. It should be noted thatat these higher pH values, the chelating agents will be significantlydeprotonated and operable for chelating metal ions. For someapplications such as, for example, the dissolution of barium scales,particularly in a pipe, tubing, or like vessel, high pH values such asabout 8 or above or about 10 or above may be beneficial in this regard.

In addition to the intended function that the chelating agent will servewhile downhole, the acid dissociation constants of the chelating agentmay dictate the pH range over which the treatment fluid may be mosteffectively used. GLDA, for instance, has a pK, value of about 2.6 forits most acidic carboxylic acid functionality. Below a pH value of about2.6, dissolution of formation cations will be promoted primarily by theacidity of a treatment fluid containing GLDA, rather than by chelation,since the chelating agent will be in a fully protonated state. MGDA, incontrast, has a pK, value in the range of about 1.5 to 1.6 for its mostacidic carboxylic acid group, and it will not become fully protonateduntil the pH is lowered to below about 1.5 to 1.6. In this respect, MGDAis particularly beneficial for use in acidic treatment fluids, since itextends the acidity range by nearly a full pH unit over which thechelating agent is an active chelant. The lower pH of the treatmentfluid beneficially may allow for a more vigorous acidizing operation totake place. For comparison purposes, the acid dissociation constant ofEDDS (2.4) is comparable to that of GLDA.

Of the biodegradable chelating agents described herein, GLDA and MGDAare currently available from commercial sources in bulk quantities witha reliable supply stream. From a supply standpoint, these biodegradablechelating agents are therefore preferred. For the reasons noted above,these chelating agents are operable over a different range of pH values,and they are complementary to one another in this respect. In additionto their pH complementarity, the biodegradable chelating agentsdescribed herein may have the capacity for selective chelate formationwith different metal ions, both as an inherent function of the chelatestability constant and a kinetic/thermodynamic formation rate as afunction of pH. In this regard, other biodegradable chelating agentsthat are less readily available from commercial sources such as, forexample, EDDS, β-ADA, IDS, and/or FIDS may be used singly or combinedwith GLDA or MGDA in order to “fine tune” the chelation properties of atreatment fluid. Other combinations of biodegradable ornon-biodegradable chelating agents may be considered as well, Table 1shows an illustrative listing of stability constants for various metalcomplexes of GLDA, MGDA, EDDS, IDS, HIDS, β-ADA andethylenediaminetetraacetic add (EDTA).

TABLE 1 Chelating Log Stability Agent Cation Constant EDTA Fe(III) 10.65EDTA Ca(II) 25.1 MGDA Fe(II) 8.1 MGDA Fe(III) 16.5 MGDA Ca(II) 6.97 MGDAMg(II) 5.8 GLDA Ca(II) 5.9 GLDA Fe(III) 11.7 EDDS Fe(III) 22.0 EDDSCa(II) 4.58 EDDS Mg (II) 6.09 IDS Fe(III) 15.2 IDS Ca(II) 6.97 IDSMg(II) 4.3-5.8 β-ADA Fe(III) 13.3-16   β-ADA Fe(II) 8.9 β-ADA Ca(II) 5β-ADA Mg (II) 5.3 HIDS Fe(II) 6.98 HIDS Fe(III) 14.36 HIDS Ca(II) 5.12As shown in Table 1, EDDS, for example, may be included in a treatmentfluid containing MGDA when a higher affinity for binding of Fe(III) isdesired and/or a lower affinity for Ca(II) binding is needed. Thecombination of MGDA and EDDS has been described for illustrativepurposes only, and upon knowing the stability constant of a givenchelating agent for a given metal cation, one of ordinary skill in theart will be able to envision an appropriate treatment fluid containingany combination of the biodegradable chelating agents for a givenapplication.

In addition to the stability constant, one of ordinary skill in the artwill recognize that the ability of a given chelating agent to react witha given cation will be a function of the treatment fluid's pH. Forinstance, the maximum chelation of Fe(III) takes place at a pH of about3 with MGDA and decreases at lower pH values. In contrast, the maximumchelation of Ca(II) and Mg(II) takes place at a higher pH with thischelating agent. Therefore, by adjusting the pH of the treatment fluid,its properties for binding a cation of interest may be altered. In theillustrative example described, a treatment fluid having a pH of about 3or below may be used to selectively remove Fe(III) cations, whileleaving Ca(II) and Mg(II) uncomplexed, thereby not wasting the chelatingagent on cations whose chelation is unwanted.

In some embodiments, the chelating agent composition may comprise about1% to about 50% by weight of the treatment fluid. In other embodiments,the chelating agent composition may comprise about 3% to about 40% byweight of the treatment fluid. In still other embodiments, the chelatingagent composition may comprise between about 3% and about 20% by weightof the treatment fluid. In some or other embodiments, the ratio of thechelating agent composition to water in the treatment fluid may rangebetween about 1% and about 50% by weight based on a known or existingconcentration. In some embodiments, the ratio of the chelating agentcomposition to water in the treatment fluid may range between about 1%and about 20% by weight based on a known or existing concentration. Insome embodiments, this ratio may range between about 3% and about 6%.

In some embodiments, the treatment fluids described herein may alsoinclude a viscoelastic surfactant. Generally, any suitable surfactantthat is capable of imparting viscoelastic properties to an aqueous fluidmay be used in accordance with the present embodiments. Thesesurfactants may be cationic, anionic, nonionic, zwitterionic oramphoteric in nature, and may comprise any number of differentcompounds, including methyl ester sulfonates (such as those described incommonly owned U.S. Pat. Nos. 7,159,659, 7,299,874, and 7,303,019 andU.S. patent application Ser. No. 11/058,611, filed Feb. 15, 2005 and nowavailable as United States Patent Application Publication 20060183646,each of which is incorporated by reference herein), betaines, modifiedbetaines, sulfosuccinates, taurates, amine oxides, ethoxylated fattyamines, quaternary ammonium compounds, any derivative thereof, and anycombination thereof. When present in the treatment fluids, thesurfactant may generally be present in an amount sufficient to provide adesired viscosity (e.g., sufficient viscosity to divert flow, reducefluid loss, suspend particulates, and the like) through the formation ofviscosifying micelles. In particular embodiments, the surfactant maycomprise between about 0.5% and about 10%, by volume, of the treatmentfluid. In more particular embodiments, the surfactant may comprisebetween about 1% and about 5%, by volume, of the treatment fluid.

When including a surfactant, the treatment fluids may also comprise oneor more co-surfactants to, among other things, facilitate the formationof and/or stabilize a foam, facilitate the formation of micelles (e.g.,viscosifying micelles), increase salt tolerability, and/or stabilize thetreatment fluid. The co-surfactant may comprise any surfactant suitablefor use in subterranean environments and that does not adversely affectthe treatment fluid. Examples of co-surfactants suitable for use in thepresent treatment fluids may include, for example, linear C₁₀-C₁₄ alkylbenzene sulfonates, branched C₁₀-C₁₄ alkyl benzene sulfonates, tallowalkyl sulfonates, coconut alkyl glyceryl ether sulfonates, sulfatedcondensation products of mixed C₁₀-C₁₈ tallow alcohols with about 1 toabout 14 moles of ethylene oxide, and mixtures of higher fatty acidscontaining about 10 to about 18 carbon atoms. In particular embodiments,the co-surfactant may be present in an amount ranging between about0.05% and about 5% by volume of the treatment fluid. In more particularembodiments, the co-surfactant may be present in an amount rangingbetween about 0.25% and about 0.5% by volume of the treatment fluid. Thetype and amount of co-surfactant suitable for a particular applicationmay depend upon a variety of factors, such as the type of surfactantpresent in the treatment fluid, the composition of the treatment fluid,the temperature of the treatment fluid, and the like. A person ofordinary skill in the art, with the benefit of this disclosure, willrecognize when to include a co-surfactant in a particular application,as well as the appropriate type and amount of co-surfactant to include.

The present treatment fluids may also optionally comprise one or moresalts to modify the rheological properties (e.g., viscosity) of thetreatment fluids. These salts may be organic or inorganic. Examples ofsuitable organic salts (or free acid forms of organic salts) mayinclude, but are not limited to, aromatic sulfonates and carboxylates(e.g., p-toluenesulfonate and napthalenesulfonate), hydroxynaphthalenecarboxylates, salicylates, phthalates, chlorobenzoic acid, phthalic add,5-hydroxy-1-naphthoic acid, 6-hydroxy-1-naphthoic acid,7-hydroxy-1-naphthoic acid, 1-hydroxy-2-naphthoic acid,3-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid,7-hydroxy-2-naphthoic acid, 1,3-dihydroxy-2-naphthoic acid,3,4-dichlorobenzoate, trimethylamrrionium hydrochloride andtetramethylamrnonium chloride. Examples of suitable inorganic salts mayinclude water-soluble potassium, sodium, and ammonium salts (e.g.,potassium chloride and ammonium chloride). Any combination of the saltslisted above also may be included in the treatment fluids describedherein. Where included, the one or more salts may be present in anamount ranging between about 0.1% and about 75% by weight of thetreatment fluid. In more particular embodiments, the one or more saltsmay be present in an amount ranging between about 0.1% and about 10% byweight of the treatment fluid. A person of ordinary skill in the art,with the benefit of this disclosure, will recognize when to include asalt in a particular application, as well as the appropriate type andamount of salt to include.

The present treatment fluids may also include one or more well-knownadditives such as, for example, gel stabilizers, fluid loss controladditives, particulates, adds, corrosion inhibitors, catalysts, claystabilizers, biocides, friction reducers, additional surfactants,solubilizers, pH adjusting agents, bridging agents, dispersants,flocculants, foarners, gases, defoamers, H₂S scavengers, CO₂ scavengers,oxygen scavengers, scale inhibitors, lubricants, viscosifiers, weightingagents, and the like. One of ordinary skill in the art, with the benefitof this disclosure, will be able to determine the appropriate type andamount of such additives for a particular application. For example, insome embodiments, it may be desired to foam a treatment fluid using agas such as, for example, air, nitrogen, or carbon dioxide.

In some embodiments, methods described herein can comprise providing atreatment fluid that comprises an aqueous base fluid, a borontrifluoride complex, and a chelating agent composition; and introducingthe treatment fluid into a subterranean formation.

In some embodiments, methods described herein can comprise providing atreatment fluid that comprises an aqueous base fluid, a borontrifluoride complex, and a chelating agent composition; introducing thetreatment fluid into a subterranean formation having a temperature of atleast about 200° F.; allowing sufficient time to pass for at least aportion of the boron trifluoride complex to form hydrofluoric acid inthe subterranean formation; and dissolving at least a portion of anyinsoluble silicon-containing compounds present in the subterraneanformation using the treatment fluid.

In some embodiments, methods described herein can comprise providing atreatment fluid that comprises an aqueous base fluid, a borontrifluoride complex, and a chelating agent composition; and dissolvingat least a portion of any insoluble silicon-containing compounds presentin a subterranean formation having a temperature of at least about 200°F. by using the treatment fluid.

In some embodiments, the subterranean formation into which the presenttreatment fluids are introduced may have a temperature of at least about200° F. As previously noted, the degradation rate of boron trifluoridecomplexes above this temperature may provide a desirably rapid rate ofhydrofluoric acid formation. As also previously described, otherhydrofluoric acid-generating compounds may be included in the treatmentfluids if the rate of hydrofluoric acid generation is insufficient. Forexample, other hydrofluoric acid-generating compounds may be included inthe treatment fluid if the formation temperature is too low to support adesired rate of hydrofluoric acid formation (e.g., at a formationtemperature of less than about 200° F.). However, it is to be recognizedthat other hydrofluoric acid-generating compounds may also be included,if desired, even when higher temperature subterranean formations arebeing treated.

In some embodiments, the present treatment fluids may be used as apre-treatment to a fracturing treatment, especially in subterraneanformations that contain different layers of sedimentary rock. In suchembodiments, the treatment fluid may be placed in a subterraneanformation via a well bore before a fracturing treatment. The subsequentfracturing treatment may be a traditional fracturing treatment or anadditional acidizing treatment directed at treating a particulate pack(e.g., proppant pack) introduced during the fracturing treatment. Insuch embodiments, the use of the treatment fluid described herein may beconsidered a prevention mechanism to prevent the formation ofpotentially problematic precipitates.

In some embodiments, a treatment fluid described herein may be used toclean the well bore area before bringing the well into final production.Using such a treatment fluid may remove damage, blockages, debris, andnatural clays in the formation, for example. In at least someembodiments, this method may be considered a remediation method of thepresent invention. For example, in some embodiments, the treatment fluidmay remediate precipitation damage that is present in the subterraneanformation.

In some embodiments, the treatment fluids described herein may be usefulin subterranean formations that comprise siliceous materials such as,for example, naturally occurring sandstone, quartz, propping material,and the like. A siliceous material may be naturally present in theformation (e.g., the sandstone) or deliberately introduced (e.g., aquartz proppant). Due to the geochemical processes operative in theformation, such as high temperature, high pressure, and abrupt changesto the geochemical balance after the introduction of treatment fluids ofdifferent ionic strength, the siliceous material may undergo rapidchanges that lead to reduction of permeability or hydraulicconductivity. When the treatment is carried out in the matrix of thesandstone, the effect is believed to remove aluminosilicates from theconductive pathways. In a particulate pack or a propped fracture, theeffects may be compounded because, under certain scenarios, geochemicalscaling may occur. Another reason is due to fines migration, which isthe displacement of particles from the rock matrix into the pack andtheir subsequent deposition. Furthermore, the presence of aluminum in asandstone or in ceramic proppants made of alumina may exacerbate theproblem due to their intrinsic reactivity in low pH media or underabrupt changes to the chemical potential of a fluid leading todissolution of the material. This signifies that varying amounts ofsilicon and/or aluminum are placed into solution, may migrate andre-precipitate or crystallize and form new minerals that obstruct theflow of fluids.

In some embodiments, it may be desirable to include a salt or a saltsubstitute in the treatment fluid. The beneficial effects of adding asalt or salt substitute are surprising, since it is conventionallybelieved that adding a salt to a treatment fluid may exacerbateprecipitation problems. A preferred example of a suitable salt isammonium chloride or like ammonium salt. It is believed that this is aproblem specific to treatment fluids containing hydrofluoric acid or ahydrofluoric acid-generating compound, since alkali metal salts such assodium and potassium salts may promote the formation of precipitates inthe presence of fluoride ions. In contrast, adding an ammonium salt maynot exacerbate the precipitation problem.

In some embodiments, the treatment fluids described herein may be usedto treat a proppant pack, particularly where the proppant pack'shydraulic conductivity has been impacted. In some embodiments, treatmentfluids described herein may be used to perform a stimulation operationin a subterranean formation, for example, an acidizing operation. Insome embodiments, the treatment fluids may be used in a fracturingoperation by introducing the treatment fluid into the subterraneanformation at a pressure sufficient to create or enhance at least onefracture therein. In other embodiments, the treatment fluid may beintroduced into a formation that has been previously fractured.

To facilitate a better understanding of the present invention, thefollowing examples of preferred embodiments are given. In no way shouldthe following examples be read to limit, or to define, the scope of theinvention.

EXAMPLES

Core flow tests were conducted using synthetic mixtures of quartz andclay. The actual synthetic mixture composition used is given for eachexperiment. The quartz employed was a mixture of 20/40 and 100 mesh in aratio of 20:80 to 40:60, where the ratio was adjusted to control thefinal permeability of the synthetic core pack. The days were obtainedfrom the Clay Minerals Society Repository. The synthetic core mixturewas equilibrated with a brine before use (typically 1 M NaCl, KCl, orNH₄Cl, but also lower concentrations were used if the clay did not showswelling characteristics). A 1-ft long, 2 in, diameter, rubber Vitonsleeve was employed to accommodate the synthetic core pack. A confiningpressure of 2000 psi and a back pressure of 500 psi-1000 psi wereapplied. Flow rates of 1 to 5 mL/min were typically employed. Sampleswere collected at regular intervals and analyzed via inductively coupledplasma spectroscopy (ICP-OES) at the equilibrium pH. A shut-in period ofone hour or more was used in each test to increase contact time of thecore pack with the treatment fluid and to increase the concentration ofdissolved ions. After the shut-in period, a brine flush of the core packwas performed. Temperature was controlled via a Eurotherm controller.

Nuclear magnetic resonance spectroscopic analyses were carried out usinga Bruker 500 MHz spectrometer. Chemical shifts were externallyreferenced to 0.1 M boric acid (δ=0 ppm) and are uncorrected. To avoidpossible contamination from borosilicate glass, apolytetrafluoroethylene NMR tube insert was used.

Example 1

The synthetic pack employed was a quartz and chlorite (CCa-1,Ripidolite, Flagstaff Hill, Calif.) mixture in a 95:5 ratio that wasequilibrated for 16-24 hours in 2% NaCl brine and maintained at 245° F.The treatment fluid contained 5% TRILON® M (sodium salt of methylglycinediacetic acid, available from BASF) and 0.17% boron trifluoridedihydrate complex at pH 3.68. A preflush treatment fluid of 15% v/vTRILON® M at pH 4 was also used (see below). Any necessary pHadjustments were made with 36% HCl or a sufficient quantity of TRILON® Mstock solution (pH 11). Several pore volumes of a like treatment fluidlacking the boron trifluoride complex were flowed through the core packbefore introducing the treatment fluid containing the boron trifluoridecomplex.

FIG. 1 shows an illustrative plot of the concentration of various ionsin fractional volumes collected from a flow test of a synthetic corepack of quartz and chlorite at 245° F. using a treatment fluidcontaining 0.17% boron trifluoride dihydrate complex and 5% TRILON® M atpH 3.68. As shown in FIG. 1, the ionic composition of the effluentindicated that aluminum was progressively eluted from the porous matrix,closely followed by magnesium and iron. The latter ion's concentrationwas the largest due to its presence in chlorite. The concentration ofboron also reflected that of the latter cations. The sodiumconcentration was nearly constant as the equilibration brine was NaCl,and the treatment fluid included the sodium salt of methylglycinediacetic acid. The amount of dissolved silicon did not exceed 500 mg/L.The test incorporated a shut-in period of 6-12 hours after introductionof the boron trifluoride complex-containing treatment fluid. Prior tothe shut-in period, the concentration of boron, from boron trifluoridedihydrate, remained relatively constant at approximately 2000 mg/L.After the shut-in period, the boron concentration decreased to nearlyhalf of its starting value and progressively increased as the post-flushbrine (NH₄Cl or NaCl or KO) began to transport the dissolved matter.

Example 2

This example was performed similarly to Example 1, except the syntheticcore pack was a 10% kaolinite/90% quartz mixture equilibrated at 240°F., and the treatment fluid contained 5% Trilon® M and 0.4% borontrifluoride dihydrate complex at a pH of 3.8.

FIG. 2 shows an illustrative plot of the concentration of silicon,aluminum, and boron in fractional volumes collected from a flow test ofa synthetic core pack containing 10% Kaolinite and 90% quartz (55% SSA-1and 45% 20/40 Brady Sand) at 240° F. using a treatment fluid containing0.4% boron trifluoride dihydrate complex and 5% TRILON® M at pH 3.8, Thefirst 10 fractions after equilibration (stage II) were collected at aflow rate of 1 mL/min. Thereafter, a 12-16 hour shut-in period wasmimicked by stopping flow. Following the shut-in period, the remainingsamples were collected at a flow rate of 1 mL/min.

The ionic profile of the effluent displayed a progressive appearance ofaluminum as the packed column was brought back to flowing conditionsfollowing the shut-in period (stage III). Stage I was performed with atreatment fluid lacking the boron trifluoride complex. The treatmentfluid used in stage II included both the chelating agent and the borontrifluoride complex, which was injected at constant rate during fivepore volumes. During stage I, the concentration of Al and Si in theeffluent was below 100 ppm for both analytes. The effluent in stage IIshowed an increase in the Al concentration from 100 ppm to 500 ppm,while the Si concentration remained nearly invariant below 100 ppm.After the shut-in period, the compositional analyses indicated aprogressive increase of both Al and Si levels (stage III). The treatmentfluid in this last stage was displaced or over-flushed with a standardbrine, and no additional treatment fluid was introduced to the packedcolumn after the shut-in period. A peak Al concentration of 1170 ppm wasmeasured. The amount of dissolved Si peaked at 148 ppm and remainedconstant around 125 ppm. Stage IV utilized additional brine to determineif boron was released over time and as the treatment fluid's pH changed.During stage III, the boron concentration ranged between approximately700 ppm and 800 ppm. Continued brine flow in stage IV decreased theboron concentration to less than 50 ppm.

Example 3

This example was performed with a synthetic core pack of 15%kaolinite/85% quartz mixture equilibrated at 245° F. The treatment fluidcontained 15% Trilon® M and 0.4% boron trifluoride dihydrate complex ata pH of 3.45. A treatment fluid containing 20% v/v TRILON® M at pH 2 andlacking a boron trifluoride complex was first flushed through the corepack (3 pore volumes), followed by a 1 hour shut-in period. After theshut-in period, the packed column was flushed with 0.2 M NaCl, returningthe pH of the effluent to 6.6. Thereafter, the treatment fluidcontaining the boron trifluoride complex (150 mL) was introduced,followed by a shut-in period of 180 minutes. Following the secondshut-in period, the core pack was eluted with 0.2 M NaCl solution.

FIG. 3 shows an illustrative plot of the concentration of silicon,aluminum, and boron in fractional volumes collected from a flow test ofa synthetic core pack containing 15% Kaolinite and 85% quartz (55% SSA-1and 45% 20/40 Brady Sand) at 245° F. using a treatment fluid containing0.4% boron trifluoride dihydrate complex and 15% TRILON® M at pH 3.45.Analyses for effluent collected before the addition of the borontrifluoride complex-containing treatment fluid are not presented in FIG.3, since no leaching of mineral phase was observed (Al and Siconcentrations <200 ppm). As shown in FIG. 3, after the second shut-inperiod, the concentrations of Al and B in the effluent increased rapidlyas they were transported by the NaCl brine. The silicon concentrations,in contrast, remained nearly invariant below 500 ppm.

Example 4 ¹¹B NMR Spectra of Boron Trifluoride Dihydrate

FIG. 4 shows an illustrative ¹¹B NMR spectrum of boron trifluoridedihydrate complex at pH 2 in a solution containing TRILON® M. FIG. 5shows an illustrative ¹¹B NMR spectrum of boron trifluoride dihydratecomplex at pH 3.8 in a solution containing TRILON® M. Based on the ¹¹BNMR spectra, it appeared that the boron trifluoride complex wasconverted to predominantly fluoroborate anion (singlet, δ=20.7 ppm) andhydroxyborate anion (quartet, d=−19 ppm). As shown in FIG. 5, a minoramount of a third boron-containing species was also formed. Based on thechemical shift, it is believed that this third species may be boric acidor another borate species with a high degree of mobility and exchange.The presence of boric acid implies that boron trifluoride can dissociateunder the treatment conditions to generate HF, HF₂ ⁻ or other activehydrogen fluoride species that can promote aluminosilicate mineraldissolution, FIG. 6 shows a comparative ¹¹B NMR spectrum of a 12.5 wt. %fluoroboric acid solution at pH 5, which shows fluoroborate anion to bethe predominate species present.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present invention. The invention illustrativelydisclosed herein suitably may be practiced in the absence of any elementthat is not specifically disclosed herein and/or any optional elementdisclosed herein. While compositions and methods are described in termsof “comprising,” “containing,” or “including” various components orsteps, the compositions and methods can also “consist essentially of” or“consist of” the various components and steps. All numbers and rangesdisclosed above may vary by some amount. Whenever a numerical range witha lower limit and an upper limit is disclosed, any number and anyincluded range falling within the range is specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues. Also, the terms in the claims have their plain, ordinary meaningunless otherwise explicitly and clearly defined by the patentee.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces. If there is any conflict in the usages of a word or term inthis specification and one or more patent or other documents that may beincorporated herein by reference, the definitions that are consistentwith this specification should be adopted.

The invention claimed is:
 1. A method comprising: combining an aqueousbase fluid, a boron trifluoride complex comprising at least one complexselected from the group consisting of boron trifluoride acetonitrilecomplex, boron trifluoride acetic acid complex, boron trifluoridedimethyl ether complex, boron trifluoride diethyl ether complex, borontrifluoride dipropyl ether complex, boron trifluoride dibutyl ethercomplex, boron trifluoride t-butyl methyl ether complex, borontrifluoride phosphoric acid complex, boron trifluoride dihydrate, borontrifluoride methanol complex, boron trifluoride ethanol complex, borontrifluoride propanol complex, boron trifluoride isopropanol complex,boron trifluoride phenol complex, boron trifluoride propionic acidcomplex, boron trifluoride tetrahydrofuran complex, boron trifluoridepiperidine complex, boron trifluoride ethylamine complex, borontrifluoride methylamine complex, boron trifluoride triethanolaminecomplex, any derivative thereof, and any combination thereof, and achelating agent composition to form a treatment fluid; introducing thetreatment fluid into a subterranean formation during a stimulationoperation; and hydrolyzing at least a portion of the boron trifluoridecomplex to form hydrofluoric acid in the subterranean formation.
 2. Themethod of claim 1, wherein the chelating agent composition comprises atleast one biodegradable chelating agent selected from the groupconsisting of glutamic acid diacetic acid, methylglycine diacetic acid,β-alanine diacetic acid, ethylenediaminedisuccinic acid,S,S-ethylenediaminedisuccinic acid, iminodisuccinic acid,hydroxyiminodisuccinic acid, polyamino disuccinic acids,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]glycine,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]aspartic acid,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]methylglycine,N-tris[(1,2-dicarboxyethoxy)ethyl]amine, N-methyliminodiacetic acid,iminodiacetic acid, N-(2-acetamido)iminodiacetic acid,hydroxymethyl-iminodiacetic acid, 2-(2-carboxyethylamino) succinic acid,2-(2-carboxymethylamino) succinic acid,diethylenetriamine-N,N″-disuccinic acid,triethylenetetramine-N,N′″-disuccinic acid,1,6-hexamethylenediamine-N,N′-disuccinic acid,tetraethylenepentamine-N,N″″-disuccinic acid,2-hydroxypropylene-1,3-diamine-N,N′-disuccinic acid,1,2-propylenediamine-N,N′-disuccinic acid,1,3-propylenediamine-N,N′-disuccinic acid,cis-cyclohexanediamine-N,N′-disuccinic acid,trans-cyclohexanediamine-N,N′-disuccinic acid,ethylenebis(oxyethylenenitrilo)-N,N′-disuccinic acid, glucoheptanoicacid, cysteic acid-N,N-diacetic acid, cysteic acid-N-monoacetic acid,alanine-N-monoacetic acid, N-(3-hydroxysuccinyl) aspartic acid,N-[2-(3-hydroxysuccinyl)]-L-serine, aspartic acid-N,N-diacetic acid,aspartic acid-N-monoacetic acid, any salt thereof, any derivativethereof, and any combination thereof.
 3. The method of claim 1, whereinthe chelating agent composition is substantially free of alkali metalsand comprises an ammonium, tetraalkylammonium, or tetraalkylphosphoniumsalt of the chelating agent.
 4. The method of claim 1, wherein thetreatment fluid has a pH of about 2 or greater.
 5. The method of claim1, wherein the treatment fluid has a pH ranging between about 2 andabout
 6. 6. The method of claim 1, wherein the subterranean formationhas a temperature of at least about 200° F.
 7. The method of claim 1,wherein the boron trifluoride complex has a concentration rangingbetween about 0.5% and about 5% by weight in the treatment fluid.
 8. Themethod of claim 1, wherein the treatment fluid further comprises ahydrofluoric acid-generating compound selected from the group consistingof fluoroboric acid, fluorosulfuric acid, hexafluorophosphoric acid,hexafluoroantimonic acid, difluorophosphoric acid, hexafluorosilicicacid, potassium hydrogen difluoride, sodium hydrogen difluoride,polyvinylammonium fluoride, polyvinylpyridinium fluoride, pyridiniumfluoride, imidazolium fluoride, ammonium fluoride, ammonium bifluoride,tetrafluoroborate salts, hexafluoroantimonate salts, hexafluorophosphatesalts, bifluoride salts, and any combination thereof.
 9. A methodcomprising: combining an aqueous base fluid, a boron trifluoride complexcomprising at least one complex selected from the group consisting ofboron trifluoride acetonitrile complex, boron trifluoride acetic acidcomplex, boron trifluoride dimethyl ether complex, boron trifluoridediethyl ether complex, boron trifluoride dipropyl ether complex, borontrifluoride dibutyl ether complex, boron trifluoride t-butyl methylether complex, boron trifluoride phosphoric acid complex, borontrifluoride dihydrate, boron trifluoride methanol complex, borontrifluoride ethanol complex, boron trifluoride propanol complex, borontrifluoride isopropanol complex, boron trifluoride phenol complex, borontrifluoride propionic acid complex, boron trifluoride tetrahydrofurancomplex, boron trifluoride piperidine complex, boron trifluorideethylamine complex, boron trifluoride methylamine complex, borontrifluoride triethanolamine complex, any derivative thereof, and anycombination thereof, and a chelating agent composition to form atreatment fluid; introducing the treatment fluid into a subterraneanformation having a temperature of at least 200° F.; allowing sufficienttime to pass for at least a portion of the boron trifluoride complex toform hydrofluoric acid in the subterranean formation; and dissolving atleast a portion of any insoluble silicon-containing compounds present inthe subterranean formation with the hydrofluoric acid.
 10. The method ofclaim 9, wherein the chelating agent composition comprises at least onebiodegradable chelating agent selected from the group consisting ofglutamic acid diacetic acid, methylglycine diacetic acid, β-alaninediacetic acid, ethylenediaminedisuccinic acid,S,S-ethylenediaminedisuccinic acid, iminodisuccinic acid,hydroxyiminodisuccinic acid, polyamino disuccinic acids,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]glycine,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]aspartic acid,N-bis[2-(1,2-dicarboxyethoxyl)ethyl]methylglycine,N-tris[(1,2-dicarboxyethoxy)ethyl]amine, N-methyliminodiacetic acid,iminodiacetic acid, N-(2-acetamido)iminodiacetic acid,hydroxymethyl-iminodiacetic acid, 2-(2-carboxyethylamino) succinic acid,2-(2-carboxymethylamino) succinic acid,diethylenetriamine-N,N″-disuccinic acid,triethylenetetramine-N,N′″-disuccinic acid,1,6-hexamethylenediamine-N,N′-disuccinic acid,tetraethylenepentamine-N,N″″-disuccinic acid,2-hydroxypropylene-1,3-diamine-N,N′-disuccinic acid,1,2-propylenediamine-N,N′-disuccinic acid,1,3-propylenediamine-N,N′-disuccinic acid,cis-cyclohexanediamine-N,N′-disuccinic acid,trans-cyclohexanediamine-N,N′-disuccinic acid,ethylenebis(oxyethylenenitrilo)-N,N′-disuccinic acid, glucoheptanoicacid, cysteic acid-N,N-diacetic acid, cysteic acid-N-monoacetic acid,alanine-N-monoacetic acid, N-(3-hydroxysuccinyl) aspartic acid,N-[2-(3-hydroxysuccinyl)]-L-serine, aspartic acid-N,N-diacetic acid,aspartic acid-N-monoacetic acid, any salt thereof, any derivativethereof, and any combination thereof.
 11. The method of claim 9, whereinthe chelating agent composition is substantially free of alkali metalsand comprises an ammonium, tetraalkylammonium, or tetraalkylphosphoniumsalt of the chelating agent.
 12. The method of claim 9, wherein thetreatment fluid has a pH of about 2 or greater.
 13. A method comprising:combining an aqueous base fluid, a boron trifluoride complex comprisingat least one complex selected from the group consisting of borontrifluoride acetonitrile complex, boron trifluoride acetic acid complex,boron trifluoride dimethyl ether complex, boron trifluoride diethylether complex, boron trifluoride dipropyl ether complex, borontrifluoride dibutyl ether complex, boron trifluoride t-butyl methylether complex, boron trifluoride phosphoric acid complex, borontrifluoride dihydrate, boron trifluoride methanol complex, borontrifluoride ethanol complex, boron trifluoride propanol complex, borontrifluoride isopropanol complex, boron trifluoride phenol complex, borontrifluoride propionic acid complex, boron trifluoride tetrahydrofurancomplex, boron trifluoride piperidine complex, boron trifluorideethylamine complex, boron trifluoride methylamine complex, borontrifluoride triethanolamine complex, any derivative thereof, and anycombination thereof, and a chelating agent composition to form atreatment fluid; introducing the treatment fluid into a subterraneanformation having a temperature of at least 200° F.; hydrolyzing at leasta portion of the boron trifluoride complex to form hydrofluoric acid inthe subterranean formation; and dissolving at least a portion of anyinsoluble silicon-containing compounds present in the subterraneanformation with the hydrofluoric acid.